Amanda,
Chankanaab park is very nice. The wife and I did the Dolphin discovery there. Never done the Passion Island Adventure. No problem about the boat load of money deal. You'll get a list of excursion's on the ship with prices and length of time each takes. Just pick from the list. If by chance you don't want to leave the ship, no problem !
Hope you have a great cruise ! No worries mate.
Happy sailing;
david